Brugia Malayi is a mosquito-transmitted filarial nematode (roundworm) parasite of humans in South and South-East Asia from India in the west to Korea in the east. It is estimated that more than 100 million people worldwide are infected by filarial parasites and more than 1 billion live in areas where filariasis is common. Most of the symptoms associated with infection appear to arise from immune reactions rather than from the worms themselves. Immune responses to the worms, which reside in the lymphatic system, lead to lymphoedema. Bouts of fever and lymphangitis are common and, rarely, a chronic infection can also cause elephantitis. Cyclophilins (Cyp) are a family of proteins binding the immunosuppressive drug cyclosporin A (CsA), a neutral 11 amino acid cyclic peptide originally isolated from the fungus Tolypocladium inflatum. The Cyp/CsA complex supresses the immune system by inhibiting T-cell activation. Using crystals of CypBm grown at 4 C, we have demonstrated diffraction to a resolution of 1.05 E. We expect to collect data beyond this resolution in the near future. We have successfully grown crystals of CypBm complexed with CsA. Although the crystals are generally small (dimensions = 150 5m), intergrown and poorly-diffracting (limiting resolution around 3.0 E), data was successfully collected on a cryogenically frozen crystal at BL9-1 to a resolution of 2.5 E. The crystals belong to space group P21 with cell dimensions a=62 E, b=100 E, c=134 E, and _=94 . Completeness to 2.5 E was 97%, I/?(I) at 2.5 E was 1.6 and the overall multiplicity was 2.9. The structure, with 8 molecules of CycBm and 8 molecules of CsA in the asymmetric unit, was solved by molecular replacement using the known structure of CycBm. Refinement of the complex structure is in progress. The current crystallographic R-factor for a model including 8 CypBm molecules, 8 CsA molecules and solvent, is 24.6% for 53,370 reflections with d = 2.5 E.